1. Technical Field
The invention relates to a sense amplifier, and more particularly to a sense amplifier having selectable power modes.
2. Description of the Related Art
Programmable logic devices (PLDs) are well known in the art. A typical PLD architecture includes an array of function blocks interconnected via an interconnect matrix. Each function block includes an AND array which provides product terms to one of a series of macrocells in the function block. Each macrocell contains an OR gate into which product terms are gated, thereby providing a combinational function. This architecture is described in more detail in the 1994 Xilinx Programmable Logic Data Book on pages 3-5 to 3-8 which is incorporated by reference herein. To provide fast and accurate CMOS level signals to the OR gate, a plurality of sense amplifiers and associated bitlines are used as the AND array which outputs the product terms.
In addition to use in function block AND arrays, sense amplifiers are also used in the interconnect matrices of PLDs. In particular, XC7000 series EPLDs employ a cross-point interconnect circuit to interconnect the function blocks. The cross-point interconnect matrix includes wordlines, which are connected to PLD inputs and feedback lines from the macrocells, and bitlines. Sense amplifiers are coupled to the bitlines for generating function block input signals. Similar to the AND array of a function block, wordline signals can be "ANDed" together within the cross-point interconnect matrix before being transmitted to a function block. FIG. 1 shows such a known sense amplifier 100 having two enable lines ENA and ENA . Sense amplifier 100 is turned on or enabled by applying a logic 0 signal to line ENA , thereby turning off N-channel transistor 101, and a logic 1 signal to line ENA. Thus, the voltage level on a feedback line FB is established by a weak pull-up N-channel transistor 103 which is turned on by the logic 1 signal provided on line ENA and a strong pull-down N-channel transistor 104 which is controlled by the signal on bitline BL. The voltage provided to a feedback line FB controls the state of N-channel transistor 102, which operates in its linear range.
A weak P-channel transistor 105 and a strong N-channel transistor 102 provides a low trigger point at a bitline voltage on the order of 0.75 volts. If the voltage on bitline BL is high, transistor 104 turns more on, thereby pulling the voltage on feedback line FB lower. This lower voltage in turn causes transistor 102 to turn more off, thereby pulling up the voltage on bitline BL less and assisting the next movement on bitline BL (i.e. the voltage going low). Conversely, if the voltage on bitline BL is low, the voltage on feedback line FB is higher. This higher voltage in turn causes transistor 102 to turn more on, thereby pulling bitline BL to a higher voltage (via transistor 105 which serves as a current source, and operates in saturation mode).
Thus, the voltage on line ABL is controlled by transistor 102 in conjunction with bitline BL. In response to the signal on bitline BL, two ratioed inverters 106 and 107 ensure that the output voltage provided on output line PT is at CMOS levels. Table 1 shows one example of logic 0 and logic 1 voltage values on the lines BL, FB, ABL, and PT.
TABLE 1 ______________________________________ Signal Logic 1 Logic 0 Difference ______________________________________ BL 0.769 v 0.746 v 0.023 v FB 1.77-1.88 v N/A N/A ABL 2.76 v 2.18 v 0.58 v NBL 0.491 v 3.33 v 2.84 v PT 4.4 v 0 v 4.4 v ______________________________________
Table 1 shows that a small swing in the voltage on bitline BL can produce a switch in the CMOS signal on output line PT.
Depending upon the states of EPROMs 121-1 through 121-n as controlled by word lines WL1 through WLn, the voltage on bitline BL is pulled lower or higher to indicate a logic 0 or logic 1 signal. If any of word lines WL1 through WLn is a logic 1 and assuming that EPROMS are erased, then the voltage on bitline BL is pulled lower to indicate a logic 0 signal. When biased on, EPROMs 121-1 through 121-n pull the voltage on bitline BL toward virtual ground VG. As shown in FIG. 1, virtual ground VG is separated from actual ground by transistor 111, which is controlled by a signal on line ABL.
Unfortunately, sense amplifier 100 draws substantial amounts of DC current in both the bitline high and bitline low states, e.g. on the order of a few hundred microamps, in addition to the large AC switching current generated during a transition in bitline state. In fact, sense amplifiers are responsible for drawing the majority of the DC current in a PLD. Therefore, a need arises for a sense amplifier that can be selectively configured for high speed signal propagation along critical signal paths, and otherwise for low power consumption along less critical signal paths.